[rc5] key security question
ted99 at ibm.net
Thu Jun 5 16:39:18 EDT 1997
On 06/05/97 Benedikt Eric Heinen <beh at icemark.ch> said:
> - Is there still a single 56bit key that can decrypt the file (i.e.
> the second encryption doesn't give any additional security)?
No, if you have 2 keys, you have 2 keys. The first pass *might* interfere
somehow with the second so that cracking the second key will be easier, but
that's extremely unlikely.
> - Do the bits just add up (i.e. the resulting ciphertext will be
> like it was encrypted using a single 112 byte key)?
Think keyspace: for each first key you have to search all the 2^56 second
keys. This makes 2^112 combinations, IOW a 112-bit key. This presumes that
using double keys does not weaken encryption (as in previous paragraph).
> - Do the keys multiply (i.e. the result would be like it was encrypted
> using a 56^2 bit key)?
> - Will the keys power each other (56^56 bit protection)?
No, that's too much, especially the last one ;-) Nobody will have to search
a bigger keyspace than was originally. Remember that the relationship
between key length and keyspace size is exponential (keyspace =
And, anyway, you don't need all this stuff because you can simply have 128
bit RC5 which will give you the protection you wanted.
/** Christ is risen ! *** __+__ ****** Fedor "Ted" Kouranov *****/
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